The electrolysis of aqueous solutions of ionizable chemical compounds such as alkali metal halide brine solutions, alkali metal phosphate solutions, hydrothalic acid solutions, and the like, in a cell equipped with an anode and a cathode separated by a porous diaphragm is well known in this art. In most instances, such cells are operated under conditions such that ionic migration and molecular migration through the porous diaphragm occurs to a substantial degree resulting in the contamination of the cathode liquor with undecomposed electrolyte and of the anode liquor with reaction products of the cathodic and anodic materials. Recent developments in electrochemical decomposition cells has lead to the replacement of the porous diaphragm member with cation active permselective membranes which substantially prevent undesirable molecular migration.
Thus in U.S. application of E. H. Cook, Jr. et al. Ser. No. 212,171, filed Dec. 27, 1971 now abandoned there is disclosed an improved method and apparatus for the electrolysis of aqueous solutions of ionizable compounds, e.g., sodium chloride, wherein the anode and cathode members are separated by a cation-active permselective membrane consisting essentially of a hydrolyzed co-polymer of tetrafluoroethylene and a sulfonated perfluorovinyl ether having the formula EQU FSO.sub.2 CF.sub.2 CF.sub.2 OCF(CF.sub.3)CF.sub.2 OCF.dbd.CF.sub.2
said copolymer having an equivalent weight of from about 900 to about 1600. Such membranes being substantially impervious to both liquids and gases, enable the production of very pure anodic and cathodic products. However it was noted that the gases formed at the anode surfaces often become entrapped between the anode and the diaphragm leading to an uneven passage of current from the anode to the cathodes.
This problem of the entrapment of gases formed at the anode has been solved at least to a major extent by the placement of the membrane on the surface of the anode facing the cathode. In copending application of E. H. Cook et al. Ser. No. 416,916 filed Nov. 19, 1973, now abandoned, methods for the emplacement of cation active permselective impervious membranes on the inner surface of porous anode members are disclosed. Thus by restricting the evolution of gases to the back face or uncovered side of the porous anode the problems of increased cell voltages due to the entrapment of such gases between the anode and membrane have been largely prevented.
In such cells wherein the anodes are constructed of valve metals such as titanium, zirconium, tantalum, niobium or an alloy thereof the surface of which is coated with an electrically conductive coating of a noble metal, such as platinum, iridium, palladium, ruthenium, osmium, rhodium, ruthenium or an alloy or oxide thereof, it was found that the proximity of the noble metal coating to the highly alkaline catholyte liquor lead to increased rate of consumption of the noble metal.
It has been disclosed, in Canadian Pat. No. 910,847, that titanium or the like valve metals can be used to construct electrodes serving a dual function of anode and diaphragm. In such components of electrolysis cells, a porous titanium base material is coated on one gas impermeable surface thereof with noble metal or oxide thereof and on the other gas impermeable surface, facing the cathode member, with an electrically non-conductive material, e.g., titanium dioxide or organic plastic material. Such an anode/diaphragm construction is permeable to liquids and gases and hardly, in the conventional sense includes a diaphragm member, much less a cation-active permselective barrier.
In British Pat. No. 1,313,441 a process for preparing chlorine and hydrogen is disclosed wherein an electrolysis cell having a cationic selectively permeable membrane is provided to separate a porous coated valve metal anode and a cathode and wherein only the surfaces of the anode which do not face the membrane are electrically active, i.e., only such surfaces are coated with a noble metal or oxide thereof. In this disclosure the permeable membrane is positioned apart from the porous anode, and molecular migration is not prevented. Further gas accumulation on the surfaces of the anode is said to be prevented by the forced circulation of the anolyte liquor.